A known radio-frequency passive acoustic transponder system imparts a radio-frequency surface acoustic wave on a piezoelectric substrate which interacts with elements on the substrate to produce an individualized complex waveform response to an interrogation signal. These piezoelectric devices (transponder “tags”) present an identification code as a characteristic time-domain delay pattern in the signal re-transmitted from the transponder. The identification code space for these devices may be 216 codes, or more, allowing a large number of these tags to be produced without ID code re-use.
These transponder tag devices do not require a semiconductor memory nor an external electrical energy source, such as battery or capacitor, to operate.
Systems for interrogating such passive transponder tags, carrying amplitude and/or phase-encoded information, are disclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160; 4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570, all of which are expressly incorporated herein by reference. Systems of this type are commercially available from X-cyte, Inc., San Jose, Calif., USA.
Other types of passive remotely readable information bearing tags include bar codes, color codes and other types of radio-frequency devices. Some of these are disclosed in the U.S. Pat. Nos. 3,273,146, 3,706,094, 3,755,803, and 4,058,217, which are also incorporated herein by reference.
Known wireless communications systems include various cellular standards (IS-41, IS-95, IS-136, etc.) as well as so-called PCS standards and data-only standards, including Cellular Packet Data Protocol (CPDP). The Metricom “Ricochet” system provides a frequency hopping 915 MHz spread spectrum wireless local data access system. These communications standards, due to their extensive infrastructure, allow a large number of simultaneous users to communicate over separate communications channels within a relatively small band without substantial mutual interference. Therefore, communications channels may be appropriated for near real time communications needs, such as voice and navigational data.
An inventory tracking system commercially available from Savi, Inc., of Sunnyvale, Calif., permits the location of tagged items within a confined area, such as a warehouse. With this system, an active, battery-equipped tag is placed on each item (e.g. container) and a special purpose interrogator transmits to all such tags within a defined area.
Further, commercial paging systems are known which transmit a paging “address” via satellite to a large area or region of the globe to identify a particular paging receiver within this region. Such a system then transmits commands and alphanumeric information to the paging receiver which is addressed. The paging receiver or “pager” can beep, vibrate or otherwise call attention to the fact that it has been addressed, and will display the alphanumeric information or issue simulated voice sounds corresponding to this information.
Finally, a number of different systems are known which can be utilized to locate an item in space. For example, the omnidirectional transmission of an easily detectable signal, such as an RF, infrared or ultrasound signal, has been used to locate the source of transmission by triangulation. Such known systems are either less than accurate or are relatively expensive.
Notwithstanding these known technologies for “tagging” and “locating” various items, such as part containers, files and even people, no system is known which allows such items to be tagged, detected and located to within a few feet, within a substantially large coverage area, e.g. for inventory control, asset management, personnel security or the like.